Direct methanol fuel cells are solid polymer fuel cells wherein methanol is used as a fuel to generate electric power, and are expected to be used as power sources for notebook size personal computers, PDAs, cellular phones, and so on. Direct methanol fuel cells have, as their center, a structure called a membrane electrode assembly (MEA), wherein a pair of electrodes are jointed to both faces of a proton exchange membrane. A methanol aqueous solution is supplied to one of the electrodes, and an oxidizing gas such as air is supplied to the other, whereby the structure can be operated as a cell. As the concentration of the methanol aqueous solution is higher, the energy density becomes higher; therefore, the cell can be driven for a long time and its fuel tank can be made small-sized. Thus, the cell is suitable for practical use.
About a polymeric membrane used in a water electrolysis cell or a fuel cell as an example of an electrochemical device wherein instead of a liquid electrolyte a polymeric solid electrolyte is used as an ion conductor, it is indispensable that the membrane has, as a cation exchange membrane, sufficiently chemical, thermal, electrochemical and mechanical stabilities as well as proton conductivity. For this reason, a perfluorocarbon sulfonic acid membrane, a typical example of which is “Nafion (registered trade name)” manufactured by Du Pont in USA, has mainly been used as a membrane which can be used over a long term. However, when the Nafion (registered trade name) membrane is used in a fuel cell wherein methanol is used as a fuel, there is remarkably caused a problem called a methanol crossover, which is a problem that methanol permeates into the Nafion (registered trade name) membrane to flow into the side of its air electrode. Thus, there arises a problem that the performance thereof as a cell falls. Furthermore, it is pointed out that an excessively high cost for the membrane hinders the establishment of fuel cell technique thereof. Accordingly, a low-concentration methanol aqueous solution has been used to restrain the methanol crossover into a minimum level. Consequently, the energy density becomes low and further the fuel tank becomes large-sized. Thus, this hinders the practical use.
One approach for overcoming such drawbacks is the development of a membrane wherein a methanol crossover is less caused. Various investigations have been made about, for example, an aromatic hydrocarbon based polymeric electrolyte membrane wherein a sulfonic acid group is introduced into a fluorine-free aromatic-ring-containing polymer, and a polymeric proton exchange membrane wherein a sulfonic acid group is introduced into a hydrocarbon based aromatic-ring-containing polymer. It is considered that the polymer skeleton thereof is advantageously a skeleton wherein a main chain skeleton has an aromatic ring structure, considering heat resistance and chemical stability. Thus, known is a skeleton wherein a sulfonic acid group is introduced into polyarylene ether, polyarylene, polyimide or some other polymer. Aromatic polyarylene ether compounds, such as aromatic polyarylene ether ketones and aromatic polyarylene ether sulfones, can also be expected as promising structures. Thus, the following are reported: a sulfonated polyaryl ether sulfone (see, for example, Journal of Membrane Science (the Netherlands), 1993, vol. 83, pp. 211-220 (Non-patent Document 1)), a sulfonated polyetheretherketone (see, for example, JP-A-6-93114 (Patent Document 1)), sulfonated polystyrene, and others.
However, the sulfonic acid group of these polymers, which is introduced onto their aromatic ring by sulfonation reaction, generally tends to be easily eliminated by heat. As the manner for canceling this, the following is reported: a monomer wherein a sulfonic acid group is introduced onto an electron-withdrawing aromatic ring is used and the monomer is polymerized to produce a thermally stable sulfonated polyaryl ether sulfone based compound (see, for example, US-A-2002/0091225 (Patent Document 2)), or a sulfonated polyarylene ether based compound (see JP-A-2004-244437 (Patent Document 3)).
A method for polymerizing a sulfonated monomer to give a sulfonated polymer directly is suggested in, for example, Patent Document 2, WO 2003/095509 Pamphlet (Patent Document 4), WO 2004/033534 Pamphlet (Patent Document 5), and WO 2004/086584 Pamphlet (Patent Document 6). The proton exchange membranes made of these polymers generally have a smaller methanol permeability value than perfluorocarbon sulfonic acid membranes. Thus, they are expected as materials promising for a direct methanol fuel cell. In membranes having high methanol permeability, such as a perfluorocarbon sulfonic acid membrane, power generating performance is not easily exhibited unless a diluted methanol aqueous solution is used. However, if a high concentration solution can be used, the system can be made compact so as to give a higher convenience. Aromatic hydrocarbon based polymeric membranes tend to exhibit a better power generation characteristic than perfluorocarbon sulfonic acid membranes. However, proton conductivity and methanol blocking property are in general properties incompatible with each other; thus, if the proton conductivity is made preferential, the methanol permeability becomes high so as to cause a fall in the power generation characteristic easily, and if the methanol blocking property is made preferential, the resistance of the membrane becomes high, thereby causing a fall in the power generation characteristic easily. For this reason, when a membrane wherein importance is attached to proton conductivity is used in an aromatic hydrocarbon based polymeric membrane also, methanol permeation from the fuel electrode to the counter electrode increases if the concentration of the fuel methanol aqueous solution is made high. As a result, the power generation characteristic does not become sufficient.
As an example wherein a proton exchange membrane as described above is applied to a direct methanol fuel cell, J. E. McGrath et al. of Department of Chemistry and Materials Research Institute in Virginia Polytechnich Institute and State University reports that a fuel cell having a relatively good proton conductivity and initial power generation characteristic was obtained. However, in this case also, the methanol concentration used in the direct methanol fuel cell is small, so that the above-mentioned problem is not solved. A cause therefor would be that as the methanol concentration is raised, the proton exchange membrane swells easily so that the electrode is peeled.    Patent Document 1: JP-A-6-93114    Patent Document 2: US-A-2002/0091225    Patent Document 3: JP-A-2004-244437    Patent Document 4: WO 2003/095509 Pamphlet    Patent Document 5: WO 2004/033534 Pamphlet    Patent Document 6: WO 2004/086584 Pamphlet    Non-patent Document 1: Journal of Membrane Science (the Netherlands), 1993, vol. 83, pp. 211-220